Work around a partition scheme selection bug on aarch64
    
    Sometimes on aarch64 clicking the partition scheme drop-down
    just doesn't seem to make the menu appear, instead the button
    goes active but that's all. It's very unlikely we'll be able
    to track down why as this doesn't happen in manual testing on
    aarch64 (according to @pwhalen), so instead let's just work
    around it.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>